Gas feeding equipment having a pressure-type flow rate control device for use in semi-conductor manufacturing devices and chemical goods manufacturing equipment is disclosed in WO 99/63412 A filed earlier by the present applicants. As shown in FIG. 9, the gas feeding equipment is designed to feed gas to a process 52 through an orifice corresponding valve 51, whilst controlling the gas flow rate such that a pressure P1 upstream of an orifice 50 of more than twice the pressure P2 downstream of the orifice 50 is maintained. Particularly, the pressure-type flow rate control device 100 comprises a control valve 53 which receives gas from a gas feeding source, said orifice corresponding valve 51 installed on the downstream side of the control valve 53, a pressure detector 54 installed between the aforementioned control valve 53 and the orifice corresponding valve 51, the orifice 50 installed at the downstream side of a valve mechanism part of the orifice corresponding valve 51, and a computer control device 55 which calculates the flow rate by Qc=KP1 (K=a constant) from the pressure P1 detected by the aforementioned pressure detector 54, and, at the same time, outputs as a control signal Qy the difference between a flow rate command signal Qs and a computed flow volume Qc to a driving part 53a of the control valve 53, thus controlling the flow rate of the gas to be fed to the process 52 by controlling the opening/closing mechanism of the control valve for adjusting the pressure P1.
Referring to FIG. 9; 55a is a temperature correction circuit; 55b a flow rate computation circuit; 55c a comparison circuit; 55d an amplification circuit: 56 a gas inlet; 57 a gas outlet; 58 a gas temperature detector; 59a,59b amplifiers, and 60a,60b A/D converters.
Now referring to FIG. 9, the orifice corresponding valve 51 and the orifice 50 are represented as separate items. However, in reality, as shown in FIG. 10, the orifice 50 is integrally fixed in a gas outlet passage S2 of the orifice corresponding valve 51 to form a valve 70 which is equipped with an integral orifice 50 (hereinafter called a valve 70 with an integral orifice).
FIG. 10 is an enlarged, partial sectional view of the valve part of the afore-mentioned prior art valve 70 with an integral orifice, wherein 71 is a valve main body, 72 a valve chamber formed in the valve main body 71, 73 an inner disc fitted in the valve chamber, 74 a metal diaphragm forming a valve, 76 a seat member made of CTFE (synthetic resin), 76a an annular valve seat, 50 said orifice installed in the scat member 76 defining the valve seat, 77 a valve presser, 78 a shaft (a valve rod), 79 a spring, S1 a gas inlet passage, and S2 said gas outlet passage. Gas flowing in the direction of an arrow from the gas inlet passage S1 flows out of the gas outlet passage S2 through the orifice 50.
Particularly, the valve part of the valve 70 with an integral orifice comprises the disc-shaped inner disc 73 fixed to the bottom of the valve chamber 72 formed in the valve main body 71, the seat member 76 fitted airtightly in a seat member receiving aperture 73a formed at the centre of the inner disc 73, the metal diaphragm 74 provided above the seat member 76, the valve presser 77 which presses the diaphragm 74 from above, and the like.
In addition, at the outer periphery of the inner disc 73, there is formed a gas inlet 73b for communication with the gas inlet passage S1. Gas flows into an air space under the diaphragm 74 through said gas inlet 73b. 
The annular valve seat 76a is formed on an upper surface of the seat member 76. Furthermore, the orifice 50 is formed in the gas outlet passage S2 in communication with the valve seat 76a. 
FIG. 11 illustrates another example of the seat member 76 (made of PCTFE) in the aforementioned FIG. 10, wherein an annular valve seat 76a is formed on the upper side of the scat member 76 substantially in the shape of a disc. The reverse side of the disc-shaped seat member 76 is formed with a concave recess having a conical shape to make thin the centre of the seat member 76, such that the small orifice 50 is formed in the central thin part.